Field of the Invention
The present invention relates to an endoscope objective lens mounted on a distal end portion of an endoscope and an endoscope provided with the endoscope objective lens.
Description of the Related Art
Recently, in the field of endoscopes, the pernasal type has been put into practical use in addition to the peroral type, and the outer diameter of the distal end of the insertion section has been reduced to about 9 mm for the peroral type and to less than 6 nm for the pernasal type in order to reduce the burden on the patient. Along with this, downsizing of the imaging unit mounted inside of the distal end portion has become essential.
Generally, endoscopes include a cylindrical rigid section with a length of about 20 to 30 mm at the distal end portion and the orientation of the rigid tip section in a body cavity is changed by performing a bending operation of a flexible section coupled to the posterior end of the rigid section. An imaging unit formed of an endoscope objective lens and an image sensor is mounted inside of the rigid tip section and an image of an observation region is captured through an objective window formed in the rigid tip section. Each end of a light guide fiber, an air and water feed tube, a forceps tube, and the like is further coupled to the rigid tip section to allow illumination of an observation region, washing and drying of the objective window, treatment of an affected area or sample collection with an appropriate treatment tool through each corresponding opening.
As an objective lens used for such endoscope as described above, the present applicant has already disclosed an endoscope objective lens described in Japanese Unexamined Patent Publication No. 2004-205779. This endoscope objective lens is configured with an importance on a long back focus in order to allow an optical path bending prism and other optical members, such as filters, including a low-pass filter, a infrared light cut filter, and the like, a cover glass, and the like to be inserted between the last lens surface and the image plane, because it may sometimes be advantageous for diameter reduction to laterally arrange the image sensor by bending the optical path of the lens system.